In many measurement applications, strains or deformations due to bending loads are significantly greater than strains or deformations due to axial loads. Since load cells are generally designed to measure the latter, the presence of bending loads complicates and often compromises the ability to accurately measure axial loads. As a result, even though load cells may be positioned along a longitudinal axis of a member, the resulting load measurement is typically a composite measurement. That is, the bending load may be contributing to the load measurement and introduce significant errors in the load measurement.
These errors can be of particular concern in applications that rely on accurate axial load measurements for control. Such applications can include, among many others, control of lower limb prosthetic, orthotic, or robotic devices. In some control methodologies for lower limb prosthetic, orthotic, or robotic devices, measurements of axial load are often used, alone or in combination with other measurements, to provide device control. However, when bending loads begin to introduce errors into the axial load measurements, the control system may over- or under-compensate in response to such errors. Accordingly, not only may the behavior of such devices no longer approximate natural motion of a lower limb, but may reconfigure the device inappropriately. In the case of prosthetic or orthotic lower limb devices, such inappropriate reconfiguration can result in loss of balance, stumbling, or injury to the user.
In view of such issues, these types of devices are typically fitted with additional sensors in an effort to more accurately determine loads and more precisely control the devices. However, while this addresses the limitations of load cells for such applications, the increase in complexity reduces the robustness of such devices and increases their assembly and maintenance costs.